1. Field of the Invention
The present invention relates to a semiconductor laser device and an optical information recording apparatus provided therewith, and more particularly to a semiconductor laser device having a Gaussian-like far-field pattern in the vertical direction and an optical information recording apparatus provided therewith.
2. Description of Related Art
There have been fabricated prototypes of semiconductor laser devices that emit light in a region ranging from blue to ultraviolet by the use of a nitride semiconductor material as exemplified by GaN, InN, AlN, and mixed crystal semiconductors thereof (see Applied Physics Letters 69, pp. 4056-4058). This semiconductor laser device has the following layers laid one on top of another in the order mentioned on an n-type GaN layer (3 μm): an n-type In0.05Ga0.95N buffer layer, an n-type Al0.05Ga0.95N clad layer (0.5 μm), an n-type GaN optical waveguide layer (0.1 μm), an In0.2Ga0.8N/n-type In0.05Ga0.95N triple quantum well active layer (In0.2Ga0.8N/n-type In0.05Ga0.95N=40 Å/80 Å×3MQW), a p-type Al0.2Ga0.8N layer (200 Å), a p-type GaN optical waveguide layer (0.1 μm), a p-type Al0.05Ga0.95N clad layer (0.5 μm), and a p-type GaN contact layer (0.2 μm). Part of the active layer is etched, from the topmost surface thereof down to the p-type Al0.05Ga0.95N clad layer so as to leave a ridge-shaped stripe with a width of 2 μm, and then, on top thereof, an electrode is formed. This semiconductor laser device has an optical waveguide structure in which the active layer and the optical waveguide layer are sandwiched between the clad layers. Light emitted from the active layer is confined within the optical waveguide structure, and lasing occurs.
However, the above-described conventional semiconductor laser device has the following problem. When the inventors of the present invention produced the above-described semiconductor laser device and examined the shape of a far-field pattern (FFP) in the vertical direction, they found out that the intensity at a base portion of the FFP thus obtained was higher than that of a Gaussian fitting curve. This leads to lower coupling efficiency at which a semiconductor laser device having such an FFP is coupled to a pickup optical system, because, when light emerging from a semiconductor laser device is coupled to a pickup optical system, only light within the range of angles at which light intensity is several tens percent or more of the peak FFP intensity is used, and light at angles at which light intensity is lower is not used. Therefore, the semiconductor laser device needs to be operated at an increased operating optical output, making it necessary to meet stricter specifications, resulting in lower yield and higher cost. Accordingly, it is necessary to reduce the light intensity at a base portion of the FFP. In other words, it is necessary to make an FFP shape closer to a Gaussian shape.
To make an FFP in the vertical direction closer to a Gaussian shape, in another example of the conventional semiconductor laser device, a so-called graded structure is proposed in which the refractive index is continuously varied from a clad layer to an active layer. This graded structure can be produced, through crystal growth, by substantially continuously varying the mixed crystal ratio of a semiconductor.
On the other hand, in still another example of the conventional semiconductor laser device, there is proposed a structure in which a clad layer or an optical waveguide layer is composed of two or more layers of different refractive indices so distributed as to be increasingly high toward an active layer. This structure too makes the shape of an FFP in the vertical direction closer to a Gaussian shape with the same effect as the graded structure described above.
However, we have examined these two structures described above and have found out that both are only marginally effective at making the shape of an FFP in the vertical direction closer to a Gaussian shape, and the effect is not dramatic.
As described above, in the conventional semiconductor laser device, the shape of an FFP in the vertical direction greatly differs from a Gaussian shape, and coupling efficiency to a pickup optical system is low. This increases an operating optical output at which the semiconductor laser device needs to be operated, making it necessary to meet stricter specifications. This results in lower yield and higher cost.